LSIT Geodesy and Control Surveys 5 — Questions and Answers
Question 1: The scale factor at the central meridian of a Transverse Mercator projection used in State Plane is typically:
- Exactly 1.0000
- Greater than 1.0000
- Less than 1.0000 (Correct answer)
- Variable depending on latitude
Correct answer: Less than 1.0000
To minimize overall distortion, SPCS Transverse Mercator zones use a scale factor slightly less than 1 (e.g., 0.9999) at the central meridian.
Question 2: Which of the following best defines the term 'geoid undulation' (N)?
- The difference between astronomic and geodetic latitude
- The vertical distance between the geoid and the reference ellipsoid (Correct answer)
- The correction applied to spirit-leveled heights for Earth's curvature
- The angular separation between the geoid and a local datum
Correct answer: The vertical distance between the geoid and the reference ellipsoid
Geoid undulation N is the signed vertical distance from the ellipsoid surface to the geoid surface at any point.
Question 3: In differential spirit leveling, 'rod readings' are taken as backsights and foresights. The elevation of a turning point is calculated as:
- HI − backsight
- HI − foresight (Correct answer)
- Foresight − backsight + HI
- Backsight − foresight
Correct answer: HI − foresight
Elevation of the next point = Height of Instrument (HI) − foresight rod reading, where HI = previous elevation + backsight.
Question 4: When computing a geodetic azimuth from an astronomic azimuth, the correction applied is called:
- Magnetic declination
- Laplace correction (Correct answer)
- Arc-to-chord correction
- Combined scale factor
Correct answer: Laplace correction
The Laplace correction accounts for deflection of the vertical and converts an astronomic azimuth to a geodetic azimuth.
Question 5: A GPS observation yields an ellipsoidal height h = 312.45 m. The GEOID12B undulation N at that point is −28.37 m. What is the NAVD 88 orthometric height?
- 284.08 m
- 340.82 m (Correct answer)
- 283.71 m
- 341.19 m
Correct answer: 340.82 m
Using H = h − N: H = 312.45 − (−28.37) = 340.82 m, because N is negative (geoid below ellipsoid).
Question 6: In GNSS control surveys, 'PDOP' stands for Position Dilution of Precision. A PDOP value of 1.5 compared to 4.0 indicates:
- Worse satellite geometry at 1.5
- Better satellite geometry at 1.5 (Correct answer)
- Higher multipath error at 1.5
- Longer observation session needed at 1.5
Correct answer: Better satellite geometry at 1.5
Lower PDOP values indicate better satellite geometry and thus more precise position solutions; PDOP < 2 is considered excellent.
Question 7: The Bowring method in geodesy is used to solve which type of problem?
- Converting ECEF Cartesian coordinates to geodetic latitude, longitude, and height (Correct answer)
- Computing the combined scale factor for a map projection
- Determining the geoid undulation from gravity observations
- Transforming between NAD 27 and NAD 83
Correct answer: Converting ECEF Cartesian coordinates to geodetic latitude, longitude, and height
Bowring's method is an iterative formula for converting Earth-Centered Earth-Fixed (X, Y, Z) Cartesian coordinates to geodetic (φ, λ, h) coordinates.
LSIT Geodesy and Control Surveys 5
The scale factor at the central meridian of a Transverse Mercator projection used in State Plane is typically: